The present invention relates to a technique for preventing the conduction of electrical current through a molten salt film. More particularly the present invention relates to a technique for preventing short-circuiting of a cell where electrolysis is being carried out in a molten salt bath.
A background patent showing the electrolysis of e.g. aluminum chloride in a molten salt bath is U.S. Pat. No. 3,893,899 of M. B. Dell et al., issued July 8, 1975, for "Electrolytic Cell for Metal Production". Such a cell is advantageously contained in a metal outer shell. Additionally it can be advantageous to tap such a cell by inserting into it a vacuum tapping tube by which molten metal collected in its sump is removed by suction. It is important in such and similar cases that the voltages actually being used for electrolysis not be able to run electrical current through the container or through the molten metal being tapped. If this occurs, energy is wasted which could otherwise be used for electrolysis. Additionally it is possible, for instance, for portions of the metal container to become red hot and thus dangerous if electrical leakage occurs.
British Pat. No. 687,758 of H. Grothe, published Feb. 18, 1953, shows a vacuum tapping tube 8 used in the tapping of molten aluminum product from a cell generating the aluminum by the electrolysis of aluminum chloride in a molten salt bath.
One of the problems inherent in operation of a cell as set forth in U.S. Pat. No. 3,893,899 is that portions of the molten salt bath and the compound being electrolyzed, e.g. aluminum chloride, can vaporize from the molten bath and condense as molten salt films on the interior lid areas of the cell. Thus a typical operating temperature in such a cell can be over 700.degree. C, and vaporization can be considerable.
In general it has been the experience that it is usually a practical impossibility to keep the metal shell of a cell such as in U.S. Pat. No. 3,893,899 completely isolated. It usually tends to become anodic, i.e. it assumes the voltage of the anode in the process. During tapping, there is a continuous path of metal from the sump up through, for instance, the lid of the cell and this entire path is essentially at the cathode potential existing in the bottom of the cell. The presence of the molten salt films on the interior surfaces of the lid can lead to short-circuiting of the lid, at anode potential, to the metal being tapped, at cathode potential. This has happened in experiments with U.S. Pat. No. 3,893,899. Extremely high temperatures can result and of course electrical energy is wasted.